A coupled DEM and LBM model for simulation of outbursts of coal and gas

An outburst of coal and gas is a major hazard in underground coal mining. It is generally accepted that an outburst occurs when certain conditions of stress, coal gassiness and physical–mechanical properties of coal are met. Outbursting is recognized as a two-step process, i.e., initiation and development. In this paper, we present a fully-coupled solid and fluid code to model the entire process of an outburst. The deformation, failure and fracture of solid (coal) are modeled with the discrete element method, and the flow of fluid (gas and water) such as free flow and Darcy flow are modeled with the lattice Boltzmann method. These two methods are coupled in a two-way process, i.e., the solid part provides a moving boundary condition and transfers momentum to the fluid, while the fluid exerts a dragging force upon the solid. Gas desorption from coal occurs at the solid–fluid boundary, and gas diffusion is implemented in the solid code where particles are assumed to be porous. A simple 2D example to simulate the process of an outburst with the model is also presented in this paper to demonstrate the capability of the coupled model

Revisiting rolling and sliding in two-dimensional discrete element models

It has long been recognized that the rotation of single particles plays a very important role in simulations of granular flow using the discrete element method (DEM). Many researchers have also pointed out that the effect of rolling resistance at the contact points should be taken into account in DEM simulations. However, even for the simplest case involving two-dimensional circular particles, there is no agreement on the best way to define rolling and sliding, and different definitions and calculations of rolling and sliding have been proposed. It has even been suggested that a unique rolling and sliding definition is not possible. In this paper we assess results from previous studies on rolling and sliding in discrete element models and find that some researchers have overlooked the effect of particles of different sizes. After considering the particle radius in the derivation of rolling velocity, all results reach the same outcome: a unique solution. We also present a clear and simple derivation and validate our result using cases of rolling. Such a decomposition of relative motion is objective, or independent of the reference frame in which the relative motion is measured

Biochar increased soil respiration in temperate forests but had no effects in subtropical forests

© 2017 Elsevier B.V. As a climate change mitigation strategy, biochar application to soil has been demonstrated to increase soil carbon (C) sequestration and reduce greenhouse gas (GHG) emission. Although numerous manipulative studies have been conducted, it is still not fully understood how biochar application affects soil respiration (Rs) and its components (i.e., autotrophic [Ra] and heterotrophic respiration [Rh] ) in forest ecosystems, especially in subtropical forests. In this study, we performed a meta-analysis of forest ecosystems and a field experiment with biochar amendments of 0, 10, and 30 t ha −1 in a subtropical forest in Zhejiang, China to examine the effects of biochar application on Rs and its components. Our results showed that biochar application significantly increased Rs by 20.92% at the global scale with an increase of 20.25% in temperate forests and a nonsignificant effect in subtropical forests. Responses of Rs to biochar application varied with experimental methods and soil textures. Similarly, our field experiment showed that biochar amendment did not significantly affect Ra, Rh, and Rs in a subtropical forest in Eastern China. Specifically, the average Rs under biochar amendments of 0, 10, and 30 t ha −1 were 2.37, 2.06 and 2.15 μmol m −2 s −1 , respectively (P > 0.05). Both Rs and Rh were positively correlated with microbial biomass C (MBC) and negatively with dissolved organic C (DOC). Both apparent temperature sensitivity (Q 10 ) of Rh and Rs were significantly higher under biochar treatments than in the control. Our findings indicate the importance of the differential effects of biochar application on Rs in different forest types for C sequestration, which may inform ecosystem and regional models to improve prediction of biochar effects on forest C dynamics and climate-biosphere feedbacks